Light-sensitive lithographic plates requiring no dampening solution have a silicone rubber layer as a top layer of the ink repellent non-image part, see, for example, U.S. Pat. Nos. 3,511,178 and 3,865,583, Japanese Patent Application (OPI) No. 94503/73 and British Pat. Nos. 1,146,618 and 1,419,643. The term "OPI" as used herein refers to a "published unexamined Japanese patent application".
In order to form an image on these light-sensitive lithographic plates requiring no dampening solution they are exposed to active rays through an original. In such a case, it is necessary to bring the original in close contact with the surface of the light-sensitive lithographic plate requiring no dampening solution so as to faithfully reproduce the original. For this purpose, a process which comprising placing the light-sensitive lithographic plate requiring no dampening solution and the original in piles between a rubber sheet and a press glass and making vacuous the space between the rubber sheet and the press glass to being them in close contact (hereinafter, referred to as a "vacuum contact process") is generally used.
However, in the prior light-sensitive lithographic plates requiring no dampening solution having a silicone rubber layer of the top layer as an ink repellent layer, since the surface thereof is very smooth and the silicone rubber is very excellent in close contact, if the original film is brought in contact with the silicone rubber layer, the contact part is firmly fixed and it becomes difficult to change the position of the original film. Further, when air bubbles occur, the air bubbles coming into the central part are very difficult to remove, even if the air is sucked from the circumference of the light-sensitive lithographic plate requiring no dampening solution by the vacuum contact process. Therefore, in order to bring the original in completely close contact with the entire face of the light-sensitive lithographic plate requiring no dampening solution, a very long time is required, particularly, when it is of a large size. In an extreme case, close contact can not be attained, even if much time is taken.
Such a requirement of much time for vacuum contact is not economical because efficiency of the plate making operation is reduced. In addition, there is a fatal problem that the original can not be reproduced faithfully if exposure is carried out in such a state that close contact of the original is insufficient. When using a light-sensitive lithographic plate requiring no dampening solution having a light-sensitive layer which generates gas by exposure, the gas generated by imagewise exposure accumulates between the plate and the original without getting away and damages the close contact of them, whereby reproduction of faithful image becomes difficult.
In order to solve the former problem, it has been proposed in Japanese Patent Application (OPI) No. 55343/80 to provide a polyolefin or polyester film having an uneven face on the surface so that a smooth face thereof touches with the silicone rubber layer. However, laminating the plastic film on the surface of the plate is, of course, not only technically difficult but also economically disadvantageous. Further, if a plastic film is provided on the surface of the plate, a distance between the original and the light-sensitive image-forming layer (hereinafter, referred to as "light-sensitive layer") becomes greater in carrying out exposure. Accordingly, there is a problem that so-called indistinct printing is easily caused by an exposure apparatus in which ideal parallel rays can not be obtained, and an image faithful to the original is difficult to reproduce. In addition, in the case of using a light-sensitive layer having a property of decomposing and releasing gas by exposure, if a plastic film is provided, there is a serious problem similar to the case of bringing the original in close contact with the silicone rubber layer, i.e. gas generated by exposure pushes up the plastic film to increase the distance from the original and, consequently, indistinct printing is formed.
In order to solve such a problem, a process which comprises spraying a solution or dispersion containing a non-volatile component directly on the silicone rubber layer has been disclosed in Japanese Patent Application (OPI) No. 27150/81. However, in this process, since the aqueous solution or dispersion is merely sprayed on the surface of the silicone rubber layer and dried, a finely uneven surface sticking to the surface of the silicone rubber layer comes off by a very small force, for example, by softly touching the surface of the silicone rubber layer with a finger. Thus it does not stably stick until the printing step wherein presence of the finely uneven surface is required. Moreover, there is a new problem that the finely uneven surface remaining be comes off from the surface of the silicone rubber layer in the resin of a powder, which adheres to the original film to stain the film.
As a result of earnest studies in order to solve such problems, the present inventors have attained the follwing invention.